1. Field of Invention
This invention relates to diffractive optical devices, refractive optical devices, illumination optical system, exposure apparatus, and exposure methods. In particular, this invention relates to illumination optical system suitable for manufacturing micro-devices, such as semiconductor elements, imaging elements, liquid crystal display elements, and thin film magnetic heads, using lithography processes.
2. Description of Related Art
In typical exposure apparatus of this kind, a secondary light source is formed substantially as a planar light source composed from many light sources, via a fly's eye lens that functions as an optical integrator. A light beam from the secondary light source (the light beam from the secondary light source actually is composed of multiple light beams from the many light sources that form the secondary light source) enters a condenser lens after being limited through an aperture diaphragm positioned near a back focal plane of the fly's eye lens.
The light beams collected by the condenser lens superimposingly illuminate a mask on which a predetermined pattern has been formed. The light that has passed the pattern on the mask forms an image on a wafer through a projection optical system. As a result, a mask pattern is projected and exposed (transferred) onto the wafer. The pattern formed on the mask is highly integrated. Thus, it is desirable to obtain a uniform illumination distribution on the wafer in order to accurately transfer the minute pattern onto the wafer.
Accordingly, a technology that forms a circular secondary light source on the back focal plane of the fly's eye lens and varies coherency σ (σ=diameter of aperture diaphragm/diameter of pupil of the projection optical system, or σ=numerical aperture on the exit side of the illumination optical system/numerical aperture of the entrance side of the projection optical system) of illumination by changing the size thereof, has been used. In addition, a technology that forms an annular or quadrupole secondary light source on the back focal plane of the fly's eye lens to improve depth of focus (DOF) and resolution of the projection optical system, has been used.
However, in the above-described conventional technologies, the cross-sectional shape of the light beam entering a point on the mask that is the illuminated surface is in the same positional relationship with respect to the two orthogonal directions on the mask in the case of the conventional circular illumination based on the circular secondary light source, as well as in the case of the conventional modified illumination based on the annular or quadrupole secondary light source (annular illumination or quadrupole illumination). In other words, in the conventional technologies, the illumination conditions are the same for the two orthogonal directions on the illuminated surface. As a result, it is not possible to realize the most optimum illumination conditions at the two orthogonal directions on the mask when the mask pattern has directionality (i.e., has characteristics/features that are different in the two orthogonal directions).
In particular, in the case of the quadrupole illumination where a quadrupole secondary light source composed from four planar light sources centered about the optical axis is formed, changing the size of all of the planar light sources and their distance from the optical axis (measured from the center of each planar light source) is disclosed in Japanese Laid-Open Patent Application No. 2001-85293 (and its corresponding European Laid-Open Application No. 1014196), for example. However, in the conventional technologies, although the size of all of the planar light sources composing the quadrupole secondary light source in the quadrupole illumination and their distance from the optical axis can be changed, various quadrupole secondary light sources have a form of two-time rotational symmetry (rotational symmetry with respect to two orthogonal axes) with respect to the optical axis by independently or continuously changing the angular position of the planar light sources.
It is proposed in Japanese Laid-Open Patent Application No. 2002-231619, for example, to form various quadrupole secondary light sources having two-time rotational symmetry with respect to the optical axis by independently or continuously changing the angular position of the planar light sources using an axicon system. However, the axicon is difficult to manufacture from fluorite, due to the limited ability to process fluorite, and thus, the manufacturing cost becomes extremely high. Accordingly, there is a demand for a technology that forms various quadrupole secondary light sources having two-time rotational symmetry with respect to the optical axis by independently or continuously changing the angular position of each planar light source using a simple structure without using an axicon system.